The present invention relates to improvements in the gaging of strain-gage transducers which will exhibit predetermined input-impedance characteristics although gaged so as to be lacking certain customary auxiliary temperature-responsive resistors, and, in one particular aspect, to unique and useful integral combinations of printed-circuit type strain gages and unstrained resistances and low-resistance connections in which the strain gages inherently offset errors induced by temperature dependency of the elastic modulus of transducer materials and in which the unstrained resistances will develop a desired normal input impedance for a transducer bridge network.
Mechanical-to-electrical transducers, in which strain gages respond to elastic deformations of elements experiencing loading, have long been known in a variety of sizes and forms, for purposes of characterizing such phenomena as force, torque, weight and pressure. In common modern constructions, they include a plurality of strain gages, such as four, which are intended to be coupled together as the impedance arms of a full electrical bridge, and which are also associated with special temperature-sensitive resistors serving to introduce compensation for unwanted temperature effects upon the transducer output. As a consequence of customer acceptance and standardizing and convenience of usage in the trade, many electrical-resistance strain gages are made with certain intrinsic values of resistance, and the related input and output impedances of bridges of which they are part, along with compensation and trimming impedances, can usually be relied upon to fall within certain limits and to match with other networks and/or devices.
In accordance with certain improvements which enable strain gages themselves to be used to offset tendencies for a transducer output to vary with temperature, the usual compensation resistors are then not needed in the inputs of bridge networks incorporating such gages, and there can be troublesome mismatching of inputs and related disturbances, particularly where bridge inputs are paralleled and not all exhibit like impedances. Additional installation and wiring of resistances to achieve remedial adjustment of input impedances involves costs and complications which are highly undesirable. However, the present invention resolves such difficulties conveniently and economically by way of printed-circuit or foil-type composite interconnected gage-and-calibration resistance units, the gages affording compensation for changes in modulus of elasticity of transducer material and the resistances calibrating the input of an intended bridge network.
A mechanical combination of a wire gage and like super-positioned and unstrained element serving strictly temperature-compensation purposes appeared rather early in the art, as shown in U.S. Pat. No. 2,344,642, and in U.S. Pat. Nos. 2,350,972 and 2,390,038 and 2,672,048 temperature-compensating filaments were introduced into composite assemblies with wire gage filaments.